The technology of forming thin-film transistors on a substrate having plasticity such as plastics is extremely important for the implementation of flexible electronic devices such as electronic paper. In particular, the techniques for producing thin-film transistors at low temperature in a simple process such as printing have received a great deal of attention because this acts as a trigger for a significant reduction in production costs. With this aim in mind, great expectations are now placed on organic thin-film transistors using as an active layer an organic semiconductor which can be produced by a coating technique such as printing.
In such an organic thin-film transistor, it is expected that the use of a thin-film of an inorganic compound such as a SiO2 thin film as an insulating layer will be adequate for the purpose of ensuring the reliability of the transistor performance.
A SiO2 thin film has been applied to various functional films, thin films for electronic devices, and the like, due to its high degree of insulating properties, dielectric strength and sealing properties, as well as its mechanical strength, abrasion resistance, chemical resistance, heat resistance, scuff resistance, optical transparency, and the like. Particularly in recent years, various functional materials using a plastic film having plasticity have been proposed, where the various uses of a SiO2 thin film have been devised for imparting the required function. Examples of the devisal include impartment of film-mar-proof properties, anti-reflection function, chemical resistance and the like.
Representative processes for producing the SiO2 thin film include thermal oxidation of silicon to form a SiO2 thin film on the surface. This is the most frequently used technique for producing a thin film for electronic devices. However, this process has a drawback that it is not applicable to the case where a silicon substrate is not used, and therefore it cannot be used in general purposes, since the substrate is limited to silicon in this process. Another common process for forming the SiO2 thin film is a process in a vacuum, representative of which includes a sputtering method or a CVD method. However, in a process using such a vacuum stage, sufficiently high surface smoothness is not necessarily obtained when producing a thicker film, thus raising the disadvantages of a decrease in insulating properties and the like resulting from the surface roughness. Further, a process in a vacuum has the disadvantage that adaptability to a large area is not always easy, leading to a significant increase in production costs.
To overcome these disadvantages, various methods of solution coating for producing the SiO2 thin film have been investigated. Sol-gel method is well known as the process for producing the SiO2 thin film through coating. However, sol-gel method requires firing at elevated temperature of 400° C. or more to form a high quality thin film, and this method also gives rise to the disadvantages that a sufficiently densified film is not easily produced and satisfactory surface smoothness is not ensured.
In these circumstances, it has been reported that when producing SiO2 by hydrolysis of a silicon compound such as a silazane compound as a starting material, a densified and high-quality SiO2 thin film can be produced. Various processes of forming a SiO2 thin film using a silicon compound as a starting material have been heretofore investigated. However, when a silicon-compound thin film is converted to a SiO2 thin film by the usual hydrolysis reaction, a firing step at high temperature of 400° C. or more is involved, and therefore resulting in the disadvantage that the SiO2 thin film cannot be produced on a plastic substrate.
Since the process using the conversion reaction of the silicon compound provides a SiO2 thin film with high film quality, it may possibly have a potential as the technology for forming the SiO2 thin film using the coating technique. For this reason, various techniques for enabling the conversion reaction at lower temperature so as to be applicable to a plastic substrate have been investigated. In order to reduce the processing temperature of the hydrolyzing process of silazane compounds as a starting material to produce SiO2, addition of a catalyst such as amines, acids, platinum, palladium or aluminum to the starting material has been reported (see patent documents 1, 2, 3, 4, 5, 6 described later). However, when the catalyst is added as described above, the added catalyst cannot be easily removed after formation of the SiO2 thin film, and it often remains in the SiO2 thin film. If the catalyst remains in the thin film as described above, the catalyst serves as an impurity, thus giving rise to the degradation of the insulating properties, when the thin film is used as a thin film for electronic elements which requires high insulating properties.
As a process for reacting the starting material at low temperature without the addition of a catalyst or the like, the process for achieving the reaction by immersing a thin film of silazane-compound in a solution containing a catalyst such as amines, acids, platinum, palladium or aluminum has been reported (see patent document 7 described later). However, also in this case, the catalyst is incorporated into the converted SiO2 thin film due to adsorption for example, since the reaction is achieved by immersion in the catalyst-containing solution. Therefore, it becomes a cause of the degradation in insulating properties and the like.
On the other hand, it has been proposed to substitute the silazane compound used for the starting material with an alkyl group and the like, and to mix a carbon-containing component in the silazane compound, in order to enhance the reactivity (see patent documents 8, 9 described later). However, when the starting material contains the carbonous component, high-temperature processing is required for the removal of the carbonous component, resulting in difficulty in producing a thin film at low temperature. Also, when a thin film is used as an electronic functional material in the state that organic components remain in the thin film, the disadvantage of reduction of the insulating properties or the dielectric strength are caused by decrease in the denseness.
In order to convert the silazane compound to SiO2 by a hydrolysis reaction, firing under conditions with an adequate humidity is indispensable. However, when producing the thin film under conditions of moisture, the moisture is adsorbed to the thin film, and in turn the adsorbed moisture becomes a cause of deterioration of insulating properties if the thin film is adapted for use in an electron element. In order to remove the adsorbed water, a process such as firing at high temperature, heating in a vacuum atmosphere or the like is still necessary. As a result, it is necessary to conduct a process step of treating at high temperature or in a vacuum atmosphere, resulting in the disadvantage of increasing the complication and the cost of the process steps.
[Patent Document 1] Japanese Patent Application Laid-open No. 6-299118 (1994)
[Patent Document 2] Japanese Patent Application Laid-open No. 6-306329 (1994)
[Patent Document 3] Japanese Patent Application Laid-open No. 7-196986 (1995)
[Patent Document 4] Japanese Patent Application Laid-open No. 9-031333 (1997)
[Patent Document 5] Japanese Patent Application Laid-open No. 9-157544 (1997)
[Patent Document 6] Japanese Patent Application Laid-open No. 11-105187 (1999)
[Patent Document 7] Japanese Patent Application Laid-open No. 7-223867 (1995)
[Patent Document 8] Japanese Patent Application Laid-open No. 6-073340 (1994)
[Patent Document 9] Japanese Patent Application Laid-open No. 7-292321 (1995)
[Patent Document 10] Japanese Patent Application Laid-open No. 59-207812 (1984)
[Patent Document 11] Japanese Patent Application Laid-open No. 60-145903 (1985)